MAGNETIC RESONANCE IMAGING OF THE CANINE OPTIC NERVE

We describe the magnetic resonance (MR) imaging aspects of normal canine optic nerve, the diameter of the optic nerve as measured on MR images, and optimal MR sequences for the evaluation of the optic nerve using a 0.2 T MR unit. Three millimeter contiguous slides of the normal canine orbital region were acquired in transverse and dorsal oblique planes using a variety of tissue weighting sequences. It was apparent that detailed anatomic assessment of the optic nerve can be performed with low‐field MR imaging, but none of the sequences provided unequivocal superior image quality of the optic nerve. The mean diameter of the optic nerve sheath complex was 3.7 mm and of the optic nerve 1.7 mm. The intraorbital and intracanalicular parts of the optic nerve are consistently visible and differentiation between the optic nerve and optic nerve sheath complex is possible using low‐field MR systems.     

A Comparison of Simple Continuous Versus Simple Interrupted Suture Patterns for Tracheal Anastomosis After Large-Segment Tracheal Resection in Dogs

Two suture techniques for tracheal anastomosis after large-segment tracheal resection were compared. Eight cartilages were resected from the trachea of each of 12 dogs; anastomoses with 4-0 polydioxanone suture were created using a simple continuous suture technique in six dogs and a simple interrupted suture technique in six dogs. Surgical time was shorter but apposition of tracheal segments at the time of surgery was less precise with the simple continuous suture technique. The dogs were evaluated for 150 days after surgery. Clinical abnormalities after tracheal resection and anastomosis were not observed. Percent dorsoventral luminal stenosis was calculated by measuring the tracheal lumen diameter on lateral cervical radiographs. Percent luminal stenosis was calculated planimetrically using a computerized digitizing tablet. Anastomotic stenosis was mild in all dogs; however, the mean percent luminal stenosis determined planimetrically was significantly greater for dogs that had the simple continuous suture technique. Planimetric measurements of cross-sectional area made before and after formalin fixation were not significantly different. Radiographic determination of percent dorsoventral luminal stenosis was a poor predictor of diminution of cross-sectional area determined planimetrically.     

Urethral Extension for Correction of Urovagina in Cattle A Review of 14 Cases

Urethral extension was performed in 14 cows with clinical urovagina and infertility. Urovagina was corrected in nine cows (64%) and was reduced in the others. Formation of fistulas at the cranial end of the extension site was the most frequent complication (4 cows). In one cow, the urethral extension was not extended far enough caudally. Eleven cows (79%) returned to fertility after surgery. Severity of urine pooling, cranial displacement of the urethral orifice, and severe preoperative vaginitis may explain the failure of the urethral extension in some cows.     

Replacement of the Medial Collateral Ligament with Polypropylene Mesh or a Polyester Suture in Dogs

The medial collateral ligament of one stifle in 20 adult dogs was excised and replaced with polypropylene mesh or a polyester suture. After 26 weeks, the fibrous tissue-prosthesis composites were evaluated clinically, morphologically, and biomechanically. Clinical lameness was not significantly different after 10 days. The polypropylene mesh reconstructions consistently had more fibrous tissue and greater collagenous ingrowth than the polyester suture reconstructions. There were four complications related to fixation of the polypropylene mesh prosthesis and one to the polyester suture. The polypropylene mesh reconstructions had greater stability and were biomechanically more similar to the natural ligaments than the polyester suture reconstructions. Although the results with polypropylene mesh were favorable, more challenging biomechanical testing and alternative anchoring techniques are required before polypropylene mesh can be recommended as a collateral ligament replacement in dogs.     

Bottom-up forcing and the decline of Steller sea lions (Eumetopias jubatus) in Alaska: assessing the ocean climate hypothesis

Declines of Steller sea lion (Eumetopias jubatus) populations in the Aleutian Islands and Gulf of Alaska could be a consequence of physical oceanographic changes associated with the 1976–77 climate regime shift. Changes in ocean climate are hypothesized to have affected the quantity, quality, and accessibility of prey, which in turn may have affected the rates of birth and death of sea lions. Recent studies of the spatial and temporal variations in the ocean climate system of the North Pacific support this hypothesis. Ocean climate changes appear to have created adaptive opportunities for various species that are preyed upon by Steller sea lions at mid‐trophic levels. The east–west asymmetry of the oceanic response to climate forcing after 1976–77 is consistent with both the temporal aspect (populations decreased after the late 1970s) and the spatial aspect of the decline (western, but not eastern, sea lion populations decreased). These broad‐scale climate variations appear to be modulated by regionally sensitive biogeographic structures along the Aleutian Islands and Gulf of Alaska, which include a transition point from coastal to open‐ocean conditions at Samalga Pass westward along the Aleutian Islands. These transition points delineate distinct clusterings of different combinations of prey species, which are in turn correlated with differential population sizes and trajectories of Steller sea lions. Archaeological records spanning 4000 yr further indicate that sea lion populations have experienced major shifts in abundance in the past. Shifts in ocean climate are the most parsimonious underlying explanation for the broad suite of ecosystem changes that have been observed in the North Pacific Ocean in recent decades.     

Alternative Labor Market Closures in a Regional Model

Five versions of a regional economic forecasting and simulation model are implemented to evaluate the forecasting accuracy and significance for impact analysis of alternative regional labor market closures. The five versions correspond to the following specifications: downward-sloped labor demand and upward-sloped labor supply, vertical labor demand and upward-sloped labor supply, an input-output version, and two general equilibrium configurations of labor demand and supply. It is found that the estimated impacts of an exogenous employment stimulus differ greatly across the model versions. Also, post-sample forecasts for 1981-1988 are run for the fifty states plus Washington D.C. with each model version to test their relative forecast accuracy. The forecast comparison shows that the general equilibrium version that specifies inelastic supply is inferior to the other versions for short-term forecasts of wage rates and long-term employment forecasts. For both short- and long-run population forecasts, the versions with completely immobile labor are more accurate than those with completely mobile labor. However, versions that specify an upward-sloped labor supply (partial labor supply adjustment) are the most accurate.     

Reduced herbicide doses in field crops: A review

Farmers are becoming increasingly interested in more comprehensive weed management programs that reduce weed populations over time and in the use of reduced herbicide doses that lower their production costs. Research indicates that there is good potential to reduce the number of herbicide applications and utilize lower herbicide doses within competitive cropping systems. Diverse crop rotations, competitive cultivars, higher crop seed rates, reduced row spacing, specific fertilizer placement, and cover crops have been identified as integral components of competitive cropping systems. This review paper explores the potential for successful use of reduced herbicide doses within competitive cropping systems that have a multiyear approach to weed management. The utilization of decision support systems or new methods of assessing active weed growth are discussed in light of further enhancing the successful use of reduced herbicide doses and advising farmers on when (and when not) they might be a viable option.     

In Vitro Pullout Strength of Screws Inserted in Adult Equine Third Metacarpal Bone After Overdrilling a 4.5-mm Threaded Insertion Hole

Objective—To determine and compare the in vitro pullout strength of 5.5-mm cortical versus 6.5-mm cancellous bone screws inserted in the diaphysis and metaphysis of adult equine third metacarpal (MCIII) bones, in threaded 4.5-mm cortical bone screw insertion holes that were then overdrilled with a 4.5-mm drill bit to provide information relevant to the selection of a replacement screw if a 4.5-mm cortical screw is stripped. Study Design—In vitro pullout tests of 5.5-mm cortical and 6.5-mm cancellous screws in equine MCIII bones. Sample Population—Two independent cadaver studies each consisting of 14 adult equine MCIII bones. Methods—Two 4.5-mm cortical screws were placed either in the middiaphysis (study 1) or distal metaphysis (study 2) of MCIII bones. The holes were then overdrilled with a 4.5-mm drill bit and had either a 5.5-mm cortical or a 6.5-mm cancellous screw inserted; screw pullout tests were performed at a rate of 0.04 mm/second until screw or bone failure occurred. Results—In diaphyseal bone, the screws failed in all tests. Tensile breaking strength for 5.5-mm cortical screws (997.5 ± 49.3 kg) and 6.5-mm cancellous screws (931.6 ± 19.5 kg) was not significantly different. In metaphyseal bone, the bone failed in all tests. The holding power for 6.5-mm cancellous screws (39.1 ± 4.9 kg/mm) was significantly greater than 5.5-mm cortical screws (23.5 ± 3.5 kg/mm) in the metaphysis. There was no difference in the tensile breaking strength of screws in the diaphysis between proximal and distal screw holes; however, the holding power was significantly greater in the distal, compared with the proximal, metaphyseal holes. Conclusions—Although tensile breaking strength was not different between 5.5-mm cortical and 6.5-mm cancellous screws in middiaphyseal cortical bone, holding power of 6.5-mm cancellous screws was greater than 5.5-mm cortical screws in metaphyseal bone of adult horses. Clinical Relevance—If a 4.5-mm cortical bone screw strips in MCIII diaphyseal bone of adult horses, either a 5.5-mm cortical or 6.5-mm cancellous screw, however, would have equivalent pullout strengths. A 6.5-mm cancellous screw, however, would provide greater holding power than a 5.5-mm cortical screw in metaphyseal bone.